FERTILISATION OF THE OVUM. 15 
appears to be the backward prolongation of the axial filament; it is occasionally 
cleft into several fibrils. 
The spiral and axial filaments consist of numerous darkly-staining fibrils. The 
former is coiled round the body and tail. The latter is continuous posteriorly with 
the end-piece, and it is continued forwards through the tail, body, and neck. In 
some animals it is said to end anteriorly in a globuloid body at the base of the head, 
but in the human spermatozoon it can be traced through the head to the “spear” 
(Von Bardeleben). 
The spear projects obliquely forwards from the apex of the head. It is a 
unibarbed process, ‘about twice as long as the head, and may be bent or curved 
upon itself. It contains an elongated body at its base, and it appears to be con- 
tinuous posteriorly with the axial filament. 
The spiral membrane is an extremely fine membrane which is attached to the 
body and tail. Its free margin is strengthened by a marginal filament. 
Spermatozoa move freely in fluid media, and each rotates upon its axis by means 
of the spiral membrane as it is propelled forwards by the lashing and contractile 
movements of the tail. 
The morphological value of the various parts of the spermatozoon is not yet 
definitely determined, but it is generally considered that the head represents a cell 
nucleus. The axial and spiral filaments are believed to consist of chromatin, and 
therefore they also are probably nuclear constituents. The protoplasmic remnant 
probably consists of cytoplasm, and the head-cap is apparently a remnant of the 
attraction sphere of the spermatid from which the spermatozoon was developed. 
Many observers have recognised the centrosome as a small structure in the 
body of the spermatozoon—possibly the globuloid body—but Nissing’s more 
recent observations upon mammalian spermatozoa have convinced him that the 
centrosome takes part in the formation of the spike; it is possibly represented 
therefore by the elongated body which Bardeleben has seen in the base of the spear 
of the human spermatozoon. 
FERTILISATION OF THE OVUM AND THE RESULTS THAT ENSUE. 
Fertilisation The mature ovum is fertilised by a spermatozoon. The two 
generative elements meet, and fertilisation takes place as a rule in the upper 
part of the Fallopian tube. The spear of the spermatozoon penetrates the zona 
pellucida of the ovum, boring through it with a rotatory motion produced by the 
spiral membrane. At the same time a conical projection, the cone of attraction, 
appears on the surface of the ovum, within the zona pellucida, directly beneath the 
point at which the spermatozoon is entering. The head, and probably a portion of 
the body of the spermatozoon, plunge into the cone of attraction ; the remainder of 
the body and the tail are cast off and disappear. The portion of the spermatozoon 
which enters the cytoplasm of the ovum is converted into a nucleus, the male 
pronucleus, which is accompanied by its attraction sphere and centrosome. When 
the male pronucleus is distinctly formed the granules of the cytoplasm in its 
neighbourhood begin to radiate around it, as if under its influence, and the 
pronucleus itself travels inwards. 
As the male pronucleus approaches the female pronucleus the latter shows 
signs of activity, it undergoes changes of form, and moves to meet the male 
pronucleus. For atime the two pronuclei lie in juxtaposition, and ultimately they 
fuse together, forming the first segmentation nucleus. 
The first segmentation nucleus is accompanied by two centrosomes which lhe at 
its opposite poles; they are the products of the male centrosome which divides as 
the pronuclei fuse. The fertilised ovum, the product of the fusion of the mature ovum 
and the spermatozoon, contains in its nucleus, the first segmentation nucleus, the 
same number of chromosomes as the primitive ovum or the sperm-mother cell, but 
the chromosomes of the segmentation nucleus are derived partly from a male and 
partly from a female individual. 
According to some authorities, both the male and female pronuclei are accompanied by 
centrosomes, and at the moment of union of the pronuclei each centrosome divides; thus four 
